1. Field of the Invention
This invention relates to a technique for the depositions by pyrolysis of a gaseous mixture, of a coating film based essentially upon oxygen, silicon and also carbon, onto the surface of a hot glass substrate. The invention further relates to the conditions in which such a deposition is carried out, a device used for this purpose, especially the nozzle that enables said gaseous mixture to be distributed and channeled, the thus coated substrate, and its applications to different types of glazing.
2. Description of the Related Art
Such techniques, using distribution nozzles for gases intended to decompose by pyrolysis on the surface of a glass ribbon, are already known, notably from French patent specification FR-2 314 152. This patent discloses a distribution nozzle placed inside the enclosure of a float bath, transversely to the axis of travel of the glass ribbon sliding over the surface of the molten metal bath. The gaseous flow emitted by the injection device of the nozzle is guided along a channel having a U section, defined by the walls of a central block flanked by "front" and "rear" heels (as defined in relation to the axis of travel of the ribbon). In this way, the gas stream undergoes a laminar flow travel over the surface of the glass parallel to this axis, during the course of which travel the pyrolysis of "precursor" gaseous elements participating in this flow takes place. Then, the thus depleted gaseous stream is removed by a suction device disposed at the outlet of the channel.
French Patent Application No. 91-01683 proposes a type of nozzle that enables the circulation of the gas stream on the surface of the glass to be optimized in order to improve the efficiency of the pyrolysis reaction, by modifying, notably, the configuration of the previously mentioned channel, notably by raising one of the heels and providing secondary suction devices.
To perform the deposition within the enclosure of the float bath requires that the gas pyrolysis device be placed inside this enclosure, generally in its "downstream part", i.e., when the glass ribbon has already acquired its dimensional stability, and this offers several technical advantages:
First of all, the glass ribbon has a temperature which is accurately known and managed throughout the length of th float bath, which ensures at the position of the pyrolysis device a glass temperature that is virtually constant and suitable for the decomposition of the gases commonly used, or in the region of 700.degree. C. In this way it becomes unnecessary to have recourse, as is frequently the case when the device is located outside the enclosure, to a reheating operation for the glass.
Furthermore, if it in intended to deposit different coating films in succession on the glass ribbon, by a plurality of pyrolysis reactions for instance, it is frequently of some advantage to be able to perform the first deposition within the float enclosure. It is thereby possible to reduce the total space requirement for the equipment in later depositions, which equipment is usually disposed between the outlet from the float bath for the glass ribbon and its entry into the annealing chamber, i.e., an oven making possible, by control of the temperature reduction, a relief of the internal stresses in the glass.
When an attempt is made, however, to apply this type of technology inside the float enclosure to the deposition of a film based upon oxygen, silicon and possibly also carbon, and the thickness of which is significant, a problem is encountered, as the state of the art demonstrates:
Thus, the patents EP-174 727 and GB-2 199 848 both propose a process for producing this type of coating in such conditions. The first document uses as "precursor" gases, that in to say gas capable of participating in the pyrolysis, a mixture consisting of a silane constituting the source of silicon and of an electron donor compound such as an ethylenic compound. If carbon dioxide is also mentioned, it is nevertheless described that it in better to avoid the use of this type of external oxygen source.
In fact, according to the above document, the sole source of oxygen that is capable of being associated with the silicon atoms arising from the decomposition of the silane is the glass itself, a certain proportion of the oxygen atoms of which would be capable of diffusing to the surface, a phenomenon that is made easier by the possible adsorption of the electron donor to said surface of the glass. According to this teaching, this capacity for diffusion is very limited, which leads to the obtaining of films of very modest thicknesses, less than 50 nm. This is why the application of these films is confined to the blocking of ion migrations. However, these thicknesses can be judged clearly inadequate for acting in an interferential manner and significantly upon the colored appearance in reflection of a substrate later provided with a functional second film, which is a very important field of application.
The second document proposes, in contrast, for the purpose of producing the same class of coating, to add to the already mentioned "precursor" gas a supplementary compound considered to fulfill the function of a source of "make-up" oxygen, such as carbon dioxide, the latter being used in a very high proportion relative to the silane. A coating having a thickness which can be as high an 80 nm is then obtained. However, the introduction of such a large quantity of an oxidizing component into a float glass enclosure leads to a high risk of perturbing the atmosphere in the latter, which has, by the well controlled proportions of hydrogen and nitrogen, a reducing character suitable for preventing any oxidation of the molten metal bath.